Several types of passive infra-red sensors have been described in the prior art for detecting intruders in high security areas. Such sensors detect the changes in the infra-red radiation falling on an infra-red detector caused by movement of the infra-red emitting intruder in the field of view of the sensor. The area under intrusion surveillance is focused onto the infra-red sensitive detector by means of an array of lenses to produce a number of discrete zones. As the intruder crosses from zone to zone, the changes in the detector output above the ambient level from the surroundings are amplified by suitable circuitry, and an alarm signal generated.
In order to maintain uniform nomenclature in describing this invention, the term "sensor" is used as far as possible to describe the complete intrusion detection unit, and the term "detector" to describe the element within it which converts the infra red radiation to an electrical signal. Since these terms are used interchangeably in the prior art and the industry in general, this delineation is not always possible, especially in quotations, and the context should always be checked when there is any doubt as to the meaning.
The effectiveness of a passive infra-red intrusion sensor system as a function of the cost of manufacture, depends to a great extent on the effectiveness of the optical system in covering the whole of the area to be protected. Various schemes for providing as full cover as possible are described in the prior art. The major problem addressed is that of providing high detection sensitivity both for intruders moving at a distance from the sensor, and for those moving close to the sensor.
In addition, a related problem is that of discrimination between signals caused by real human intruders and those caused by pets such as dogs, cats, etc., moving close to floor level around the protected areas. This problem, known as pet immunity, is especially serious, since a proliferation of false alarms resulting from pets may cause the owners to switch off an otherwise effective alarm system, and in a situation where the presence of pets may be desirable, as they may add an additional element of protection.
In U.S. Pat. No. 4,058,726, assigned to Cerberus AG of Switzerland, the inventors describe a focusing system composed of a vertical array of cylindrical lenses with vertical axes. When correctly located in front of the detector, the array divides the surveillance area up into a number of vertical strip-shaped sectorial zones fanning out from the sensor. There are a number of limitations and disadvantages to a simple array of this type. Firstly, having only one sort of lens, the sensor is unable to effectively deal both with the far and the close fields of view. In particular, since cylindrical lenses magnify in one direction only, objects distant from the sensor do not fill the lens aperture well, and the far field sensitivity is therefore low. Only a convex focusing lens, usually, for this geometry in Fresnel form, is able to give good sensitivity at the far field, and practically all such systems currently produced use convex focusing lenses for the far field.
In addition, two further disadvantages of this system have been enumerated in U.S. Pat. No. 4,740,701, which too was assigned to Cerberus A. G. The authors state that such a simple cylindrical array is unable to provide good 90.degree. vertical angular coverage when a "protective curtain" has to be provided in front of an opening such as a door or window. Furthermore, as it moves closer to the sensor, an object becomes poorly focused, both because of the non-optimal focusing distance, which has to be optimized for some median distance, and because of increased image aberrations as the object falls off-axis at angles below the sensor. Because of these two effects, to a large extent, almost all infra-red intruder detection systems suffer from reduced sensitivity in the lower fields of view which cover middle and near distances, and this is a problem which they have to solve in order to maintain good wide range coverage. At the same time, this property does have the positive effect of providing an inherent element of pet immunity for all such systems.
In the above mentioned U.S. Pat. No. 4,740,701, the inventors therefore describe a sensor which attempts to overcome these problems by using substantially cylindrical Fresnel lenses each having a longitudinal axis, a focal point and a focal length, the lenses being curved along an axis perpendicular to the longitudinal axis, to form a sector of a circle with radius equal to the focal length, and with the infra-red detector located at this focal center point. This prior art provides a well focused, narrow zone of surveillance area, which extends over a vertical angle of up to 90.degree.. It also allows coverage of more than one direction, by using a separate Fresnel lens for each direction rather than an array of lenses. However, like the previously described Cerberus focusing system, the use of only one sort of lens prevents the sensor from being able to deal effectively both with the far and the close fields of view. Furthermore, the narrowness of the surveillance zones is in itself a serious drawback for covering a larger area, and it is thus limited to situations which require only one or two protective curtains.
In U.S. Pat. No. 4,604,524, Y. and M. Kotlicki describe a sensor which overcomes some of these drawbacks, especially in the far field areas, by means of an array composed of multi-faceted convex focusing lenses of differing configurations, each configuration corresponding to differing protection zones. The arrays are spatially arranged to define several protection zones at various angles to the horizontal. The lens array is curved around a vertical axis, such that the sensor is able to cover an azimuthal angle of up to 140.degree.. The vertical angle covered is from 25.degree. above the horizontal, to 10.degree. below the horizontal. However, as is apparent from the ray coverage scheme shown in FIG. 6B of this patent, this system, too, is primarily effective for wide area coverage, and is limited in its ability to detect intruders in areas closer to the sensor. As the intruder approaches the sensor, the emitted radiation coming from the lower zone becomes more and more off-axis with respect to the normal to the sensor, and the sensitivity of the system to such radiation falls off considerably.
In U.S. Pat. No. 4,734,585 assigned to Racal Guardall, one of the leaders in the field of intruder detection, the inventor I. Owers points out that a further cause of reduced sensitivity close to the sensor, and therefore in the lower zone since the sensor is mounted high on a wall, is that the signals generated by objects moving in these areas are of high frequency, to which the detector is less sensitive. His invention attempts to fulfill the requirement of increasing sensitivity close to and at angles below the sensor by means of a composite lens array which includes some slotted sections. This array has an upper section composed of Fresnel lenses (which from their described function must be convex lenses), providing multi-zone long range coverage at the horizontal level, and two lower sections of Fresnel (convex) lenses, each curved at an increasing angle below the horizontal, to provide medium and short range coverage. The latter two sections include vertical slots between the lens segments, through which the infra-red radiation passes without any focusing action. These slots define zones with large fields of view, determined solely by the dimensions of the slots and their location with respect to the infra-red detector.
This slot technology has a number of disadvantages. The inventor himself states that because of the lack of any focusing effect, the sensor has a large field of view, which the image of an intruder may not fill. The detection sensitivity is therefore low. However, the inventor claims that for detection in the near field, the proximity of the intruder to the sensor increases the amount of radiation received, and thus should compensate for the lack of any focusing effect of the slots. This limits the effectiveness of such slots to the near field, and a maximum detection distance of 4 meters is quoted in this patent.
Furthermore, the only effective and currently used signal detection technique for passive intrusion sensor systems is with a dual element detector with differential signal analysis. However, for an array of slots, it is essential that the location of the slot array be significantly closer than the focal length of a typical optical element, such as a lens, in order to achieve zone separation. If the zones overlap too much, they cancel each other out. If such slots are integrated together with Fresnel lenses, such as described in FIG. 4 of this patent, the use of a double detector will cause excessive overlap, and a single element detector is required. This has been long recognized by the industry as being less effective and reliable than double element detection.
Finally, in order to achieve sufficient sensitivity in the absence of any focusing effects with the slot configuration, the slots have to be located very close to the detector surface, 9.5 mm in the basic embodiment described in the patent. Though the inventor claims that this makes for a compact sensor, it also complicates the construction, because the focal length of the lenses used are typically between 25 and 35 mm, therefore conflicting with the slot distance requirement.
The inventor also states that the infra-red radiation suffers lower loss as it passes through the plain strips or slots, compared to the losses suffered in passage through the Fresnel lens segments. From this it is apparent that the use of convex Fresnel lenses for the near field should be avoided because of the transmission losses they cause at the angles of incidence used for the near field.
A. Y. Messiou in U.S. Pat. No. 4,868,391, assigned to the U.S. Philips Corporation, describes a composite lens array composed of two orthogonal flat sheet assemblies of individual convex Fresnel lenses, one horizontally and one vertically disposed in front of the infra-red detector. The vertical sheet provides long range surveillance over the area to be protected, while the horizontal sheet located at its lower edge, covers the near field and the area below the sensor. This inventor too states that the use of Fresnel lenses at high angles of incidence leads to large off-axis losses, this being a characteristic of such lenses. In FIG. 4 of this patent, the inventor shows the strong fall-off in sensitivity for Fresnel lenses operating at large angles of incidence, with the radiation falling to half at 17 degrees off-axis, and to only 10% at twice this angle.
Therefore, Messiou, like Owers, also suggests that in order to increase sensitivity in the near field, for radiation coming from the lower zones where the angle of incidence is high, the convex Fresnel lenses may be replaced by clear slots, which in this embodiment, alternate with opaque strips. These clear slots are located along the front edge of the horizontal lower sheet, and the bottom edge of the vertical sheet. The radiation from a close intruder passes unfocussed through the slots to the detector. Such slots are effective for detecting motion of an intruder transverse to the direction of the length of the slots. These slots suffer though, from the same problems mentioned above regarding the Owers technology.
Messiou also proposes the use of pairs of orthogonally aligned cylindrical lenses, each lens of the pair being aligned at an opposing angle of 45.degree. to the edges of the sheets. Since each cylindrical lens focuses radiation in one direction only, such a pair of orthogonally aligned lenses effectively eliminates any preferred direction of motion along which an intruder could move without causing a change in the radiation incident on the detector. The orthogonal cylindrical lens pairs, with their defined .+-.45.degree. alignment relative to the direction of the field of view of the detector element, are specifically prescribed for directional motion discrimination, in order to detect intruders moving straight towards the sensor. With a spherical lens, such motion would cause virtually no change to the signal from the detector.
In U.S. Pat. No. 5,670,943, W. S. DiPoala et al. describe an intruder sensor designed with particular emphasis on solving the problem of pet immunity. Their invention, like most of the prior art mentioned hereinabove, and like the majority of systems currently available, divides the area to be protected into upper and lower zones, which are equivalent respectively to the far, and middle or near fields of the previously mentioned prior arts. The lower zone is that which intersects the floor plane within the protected area, and pets are generally only to be found in this lower zone. Each zone is imaged onto the detector elements by means of its own multi-faceted optics array. Pet immunity is achieved in this invention by lowering sensitivity in the lower zones and controlling temperature discrimination detection levels, such that pets do not trigger an alarm.
However, even though this is not always mentioned, the inventions described in almost all of the prior art previously mentioned also incorporate an intrinsic level of pet immunity, since, because of several opto-geometric effects, the lower zones generally have lower sensitivity than the upper zones. These effects include sensitivity reductions due to off-axis phenomena and due to the larger f-numbers needed for the focusing optics for the lower zones. Such effects are, for instance, shown explicitly in the Owers and Messiou inventions.
FIG. 6 of Owers' patent illustrates the decrease in sensitivity of a sensor, in moving from the far field to the medium field to the near field segments of its Fresnel lens array. Owers even states, inter alia, that "a sensor using focusing optical arrangements tends to have very poor sensitivity close to the sensor but good longer range characteristics". Owers also points out, as mentioned above, that there is also a reduction in sensitivity close to the sensor because of the poor high frequency response of the detector.
FIG. 4 of Messiou's patent illustrates the reduction in the radiation received at a detector due to three geometric off-axis effects of a planar lens array, namely the cos.theta. geometric effect, increased reflection from the lens array surfaces, and lens aberration and out-of-focus effects. These off-axis effects are equivalent to those arising from the location of the emitting objects in the lower zone.
In the Kotlicki's invention also, though the effect of reduced sensitivity in the lower zone is not mentioned explicitly, it is evident from FIG. 3A of their patent. The aperture of the lower zone lenses 70 is smaller than that of the upper zone lenses 60, and hence the f-number of lenses 70 larger than that of lenses 60, resulting in reduced sensitivity in the lower zone. The resulting reduced sensitivity is apparent in FIG. 6B, from the reduced range of detection for radiation from the lower zone, labeled f, as compared with that from the upper zone, labeled m.
Based on this invention, in the early Model MR-3000 and Fox passive infra-red intrusion sensors, manufactured by Visonic Limited of Tel Aviv, Israel, seven small convex lens segments were used in the lowest row of the lens array for imaging the near field. This can be observed in the instruction manuals and data sheets dated 1986 and 1987 respectively for these models. In later versions, when it became apparent that the reduced intrusion detection probability resulting from the lower near field sensitivity, was more problematic than the element of pet immunity provided by this lower sensitivity, three larger lenses, each with smaller f-number, were used in the lower row. This is shown clearly in the instruction manuals sent with these models from the end of the eighties. This provided a better near field detection capability, at the expense of pet avoidance.
In order to improve pet avoidance, these Visonic sensors incorporate a method, described for instance on page 28 of the 1987 issue of the MR-3000 instruction manual, and pages 4 and 5 of the 1990 Fox installation instructions, whereby a mask of one or two layers of infra-red attenuating material is positioned over the bottom two rows of lenses, thus reducing the transmission efficiency of the lower zone optics, and so reducing false alarm signals from pets roaming in the lower zones.
Arrowhead Enterprises Incorporated of New Milford, Conn., use a method called uniform imaging to provide pet immunity. This is described in the specification sheet for the Model IR202 passive infra-red sensor, manufactured by them for a number of years. This is effectively a method whereby the field of view is adapted to the distance from the sensor. This sensor, like most of the contemporary art, uses a faceted optics lens array for imaging a number of separate zones of the protected area. For the closer zones, where a pet or even a rat or mouse would fill much of the field of view, leading to false alarms, lenses with a different f-number are used, in order to reduce the signal produced by objects in the closer zones, thus providing pet immunity.
In U.S. Pat. No. 5,670,943, DiPoala et al., presumably without being aware of all of the above mentioned prior art, describe the use of optics in the lower zone, which are less efficient at transmitting infra-red energy than the upper zone optics. In the detailed description of the invention, a preferred embodiment shows lower zone lenses with reduced sensitivity by virtue of their smaller aperture or larger f-number, as taught in much of the above mentioned prior art. They further state that pet discrimination by reduced lower zone sensitivity can also be attained by means of defocused optics or by optical filters (as taught in the Visonic prior art), both of which are equivalent, in their words, to reduced "effective" optical aperture or greater "effective" f-number. They also propose reduced electronic amplifier sensitivity as a method to provide pet immunity.
The DiPoala invention suffers from a number of serious disadvantages. In the first place, the reduction in sensitivity is applied over the whole of the lower zone, the zone being defined by the infra-red optics or by other means used to focus or direct the radiation from it onto the sensor. The inventors state in column 5, lines 16-23, that as a result of the sensitivity reduction in the lower zone, "since household pets such as cats and dogs will not normally be present in the upper more sensitive zones, the `catch` performance of the detector is enhanced without sacrificing pet immunity". It is not clear how the `catch` performance of a system is enhanced by reducing sensitivity in the lower zone. Most of the inventions described in the recent prior art quoted, in order to fulfill the primary function of detecting intruders, strive to provide improved sensitivity in the lower zones, where the sensitivity tends to fall off of its own accord. The DiPoala invention, in order to provide pet immunity, takes a backward step to the earlier techniques which have poorer lower zone sensitivity. This approach thus foregoes the advantages of improved lower zone detection, as demanded by the latest requirements of the industry. The DiPoala mechanism could perhaps be better described as lower zone desensitization, and it may seriously affect the probability of intruder detection in the near and mid-field zones. The efficacy of this approach would thus seem to be dubious.
Furthermore, the size of the field of view in the lower zone per optical element is given as about 0.5 ft wide by 0.75 ft high, which means that a typical pet completely fills the field of view. Therefore, in order to discriminate between a human intruder, who too fills the whole field of view, and a pet in the lower zone, the invention relies on differentiating between the difference in temperature of these two subjects with respect to the background temperature. The inventors state that dogs have a temperature differential of from 2 to 6.degree. F. above a normal room temperature background of 70.degree. F., depending on the length of their hair, while a human is from 8 to 13.degree. F. above the background, depending too on his/her clothing. These emission temperature differences are very close, and are obviously also strongly dependent on environmental conditions and on which part of the intruder's body is imaged by the sensor, besides the dependence on pet hair length and intruder clothing. The particular circumstances present in each intrusion event could easily introduce errors considerably higher than the quoted minimum 2.degree. F. difference between typical human and pet emission temperature differences. The graphs shown in FIGS. 4 and 6 of the DiPoala patent, which show the human signal in the lower zone to be a factor of about 2.3 larger than the pet signal, would thus seem to show an optimum situation. In practice, the difference would generally be smaller or even reversed, as explained above.
It is therefore likely that intruder sensors based on this invention suffer either from false alarms if the alarm threshold is set too low, or from missed intrusions if the threshold is set too high. Indeed, it is possible that because of the difficulty of achieving the fine temperature discrimination required, the inventors found the need to decrease the likelihood of detecting pets by artificially reducing the detection sensitivity of the lower zones, thereby also reducing the detection probability for a human in the lower zone.
Since the detection reliability of the DiPoala invention is dependent on the difference between the intruder emission temperature and the background temperature, a reference measurement of the background temperature is required in order to increase the sensitivity of the detection system as the background temperature approaches the temperature of the human body. This is performed by a thermistor mounted in the sensor head next to the detector. However, since this location does not necessarily provide an accurate indication of the true background temperature in the immediate vicinity of the intruder, and because of the difference in the pet and human body temperatures, this is a further potential cause for false alarms or missed intrusions.
The field of view used in any intrusion sensor has a major effect on the ease with which the system detects small objects such as pets. Owers, in the above-mentioned U.S. Pat. No. 4,734,585, pointed out that if the field of view is too small, even a rat or mouse may fill it and cause false alarms. This too is the approach adopted in the Arrowhead Enterprises' "uniform imaging" technology mentioned above. Therefore, DiPoala's stated use of a small field of view would appear to acerbate the problem of pet immunity, rather than to assist in solving it.
In none of the prior art mentioned has an adequate solution been provided for the problem of the detection of intruders in all of the protected area fields, far, middle and near. Both the Messiou and Owers prior arts mentioned, which use lens arrays with transparent slots to cover the middle and near areas, suffer from poor sensitivity both because the slots lack specific focusing means, and because they may require use of a single element detector because of zone overlap. Furthermore, the problem of providing efficient pet discrimination has also not been satisfactorily solved, especially in those examples of the prior art where the lower zone sensitivity is maintained at a value high enough to detect intruders in the near field. The DiPoala invention and the masked implementations of the Visonic MR-3000 and Fox sensors, while claiming good pet immunity performance, would appear to suffer from a lack of sensitivity in the whole of the lower zone. The DiPoala invention probably also has a signal level discrimination problem, likely to result in either an excess of false alarms, or in missed intruders.
The disclosures of all publications mentioned in this section and in the other sections of the specification, and the disclosures of all documents cited in the above publications, are hereby incorporated by reference.